1. Field of the Invention
This invention relates to a method of depositing a metal on a surface and more particularly, to a photographic-like method of depositing a metal pattern on an insulative surface utilizing a sensitizer selected from the group comprising a photosensitive indium-palladium sensitizer, a photosensitive cerium-palladium sensitizer, a photosensitive nickel sensitizer, a photosensitive manganese sensitizer, a photosensitive uranium sensitizer, a photosensitive molybdenum sensitizer and a photosensitive tungsten sensitizer.
2. Description of the Prior Art
Various methods for producing metallic patterns on substrates to produce the circuit boards are similarly well known. These methods include, alone or in various combination, positive and negative printing processes, positive and negative silk-screening processes, positive and negative etching techniques, electroplating and electroless plating.
Electroless plating has found great favor with many workers in the art and has, in fact, been known in at least rudimentary form since before 1845 (see Symposium on Electroless Nickel Plating, published by the American Society for Testing Materials as ASTM Special Technical Publication, No. 265 in November of 1959).
Generally speaking, electroless plating requires a so-called catalization step during which a substrate surface to be electrolessly plated with a metal has placed thereon a material, usually a metal salt. This metal salt is capable of reducing the plated metal from an electroless bath without the use of an electrical current. Catalization by such a material is referred to as such because the materials used, usually the salts of the precious metals (palladium, platinum, gold, silver, iridium, osmium, ruthenium and rhodium) serve as reduction catalysts in the autocatalytic electroless plating process. Often, catalization is characterized as providing "nulceating sites" onto which the plated metal is "brought down" by a chemical reduction, or more generally, by a redox reaction. See, for example U.S. Pat. Nos. 3,119,709 and 3,011,920.
Refinements of the basic electroless plating technique are necessary when the plated metal is electrolessly plated onto selected portions of a substrate surface in a pattern, rather than on the entire surface, to produce a circuit board. One such refinement is the additive, photoselective metal deposition process of M. A. DeAngelo at al., U.S. Pat. No. 3,562,005.
In the DeAngelo at al, additive process of metallic pattern generation, patterns are generated without etching or photoresist masking. Specifically, a solution, called a "photopromoter" which has (or at least a part of which has) the ability to be retained on a substrate is applied to the substrate. The "photopromoters" revealed in DeAngelo et al. are solutions comprising either Sn, Ti, Pb, Fe or Hg ions. The retained photopromotor (Sn, Ti, Pb, Fe or Hg ion containing) has a photopromoter species, i.e., the respective metal ion, which is capable of changing oxidation state upon exposure thereof to appropriate radiation. In one oxidation state (but not both) the photopromoter species is able to reduce, from a salt solution thereof, a precious metal (there defined as palladium, platinum, gold, silver, osmium, indium, iridium, rhenium or rhodium). The precious metal is capable of initiating an autocatalytic plating process.
After the substrate retains some of the photopromoter, it is selectively exposed to the appropriate radiation, specifically ultraviolet radiation of short wavelength and below 3,000 A. This exposure renders some portions of the substrate able to reduce the precious metal and rendering other portions not so capable. Subsequently, electroless metal is deposited only where it is desired, i.e., on the reduced precious metal.
Some potential photopromoters do not exhibit "practical wetting" of desirable substrates. "Practical wetting" is defined as the ability of a substrate to retain, on a substantially macroscopically smooth, unroughened portion thereof, a continuous, thin, uniform layer of a liquid, such as water or other liquid medium, when the surface is held vertically, or in any other orientation. To eliminate this problem, the novel DeAngelo et al. additive process of metallic pattern generation may be employed with the novel method of J. T. Kenney, U.S. Pat. No. 3,657,003, which disclosed methods of rendering a non-wettable surface wettable, so that all the DeAngelo et al. photopromoters can be used therewith. A method of wetting a hydrophobic surface while at the same time rendering such surface photosensitive, for photopatterning thereof and for an ultimate electroless metal deposition, is desired and is an object of the subject invention.